Re: A weighty subject

[Glenn Knapp <kahuna@VCN.COM>]

Arlyn DeBruyckere wrote:

 It is also a problem in teaching them about significant figures that 40 Lb

 (1 sig fig) can be equal to 18.1 Kg (3 sig figs). They don't understand

 why I get upset when they measure 20 ml and then decide to record 20.00 ml

 because I told them they should measure to the nearest .01 ml.

I would have said that the value 40 pounds is precise to 2 sigfig. Not all

final zeros are insignificant; it depends on the context.

I agree. I'd sure complain loudly if I bought a 40 pound bag

 of potatoes and found on weighing it that I'd got 36.5 pounds.

The implied contents is net, and it should be *at least* 40.0

 pounds. This particular example also demonstrates the shallow

 nature of the "significant figure" idea. Surely Arlyn wouldn't

 tell his students that the "correct" label should read 20 kg.

 Leigh

I dunno, such real world problems can actually be useful.  In my class we discuss all the foolish mistakes that are to be found out there in the real world made by people who are unsophisticated in the ways of physics.  Why does the label say 40 pounds (18.1 kg)?  Becuase the people who sell the potatoes obviously have no concept of significant figures  --  but of course, we, who study physics, do, and so we can be amused at such foolishness.  If they really were selling 40 lbs of potatoes, what kind of scale did they use to measure the weight?  Obviously one that was calibrated to the nearest 100 lbs.   Students get extra credit for finding such foolish violations of proper physics procedures.

Glenn

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